BOSTON - A new study has found that wood-burning power plants using trees and other "biomass" from New England forests release more greenhouse gases into the atmosphere than coal plants do over time.

The report, commissioned by Massachusetts state environmental officials and conducted by the Manomet Center for Conservation Sciences, concludes that the net cumulative emissions of greenhouse gases from replacing coal-fired plants with biomass would be 3 percent greater by 2050 than from using coal to generate electricity.

Coal is considered one of the chief culprits of greenhouse gas emissions.

Researchers arrived at the figure by comparing how much carbon is emitted into the atmosphere through the burning of wood - what they termed "carbon debt" - with the amount of carbon removed from the atmosphere from the regrowth of forests, or "carbon dividends."

The report found that harvesting trees for biomass facilities could have "significant localized impacts on the landscape, including aesthetic impacts of locally heavy harvesting as well as potential impacts on recreation and tourism."

The findings of the six-month study support arguments by biomass opponents who claim that plants proposed for Greenfield and elsewhere around western Massachusetts would produce more carbon dioxide than those burning fossil fuels.

"This comports with my own research," said Mary Booth of Pelham, co-founder of the Massachusetts Environmental Energy Alliance. "Over any time frame that we care about for reducing greenhouse gas emissions, biomass emits dramatically more CO2 than any fossil fuel. It would literally take decades to get back the carbon you lost when you burned those trees, and then you're still not where you would be had you just let the forest grow, because as it grows, the forest sequesters a lot of the carbon that's emitted by fossil fuels."

Biomass has long been part of the state's portfolio of renewable energy sources, along with solar, wind and geothermal energy. The Patrick administration has already invested $1 million to jump-start four proposed wood-burning plants in Russell, Greenfield, Springfield and Pittsfield, as it tries to reach the state-mandated goal of reducing greenhouse gas emissions 80 percent by 2050.

Massachusetts Environmental Secretary Ian Bowles said Thursday the state is now rethinking that policy, including taxpayer incentives for wood-burning plants.

"Now that we know that electricity from biomass harvested from New England forests is not #carbon neutral' in a time frame that makes sense given our legal mandate to cut greenhouse gas emissions, we need to re-evaluate our incentives for biomass," he said in a statement accompanying the report.

Matthew Wolfe, principal owner of Cambridge-based Madera Energy, which is planning the $250 million Greenfield biomass generator, said he doesn't think the study "wholly represents what we're doing."

The Madera plant, Wolfe said, depends almost entirely on waste wood from storm damage, from development-related land clearing, and some of the treetops and branches left after tree harvest for more lucrative forest cutting.

In the case of the use of treetops and limbs, and nonforest sources like tree trimming, landscaping or land clearing, the study says, there could be "rapid recovery," with nearly 70 percent of the carbon losses "recovered" in one decade. Thus, all bioenergy technologies, even biomass electric power, compared to natural gas electric, look favorable when biomass "wastewood" is compared to fossil fuel alternatives."

Wolfe said, "A significant amount of our resources would come from that category. There might be a fundamental misunderstanding of what our plans are here. They assume we're going to go in and cut down a bunch of trees for a biomass plant."

Still, the state Executive Office of Energy and Environmental Affairs has launched a set of public hearings to help set a 2010 target for cutting the state's greenhouse gases, the first of which was held Wednesday in Springfield. Under the state Global Warming Solutions Act, which became law in 2007, Massachusetts is required to reduce greenhouse gas emissions across the economy 80 percent by 2050.

Already, biomass opponents are pressing the Patrick administration to do away with financial incentives to qualifying power producers to cut greenhouse gases with wood-burning plants.

Wolfe said removing those incentives "would cause problems" for financing the Greenfield project.

Bowles commissioned the study after environmental activists warned biomass power plants could add to global warming. Activists are also pushing a Massachusetts ballot question to severely restrict the amount of carbon dioxide the power plants can emit.

"The sobering conclusion is that Massachusetts cannot produce very much new energy from forest resources while also protecting the health of our forests and reducing greenhouse gas emissions," said Sue Reid, a staff attorney for the Conservation Law Foundation.